Electrochemical production of peroxopyrosulphuric acid using diamond coated electrodes

ABSTRACT

A process for the electrochemical production of peroxo-disulfuric acid and peroxo-disulfates is provided. In the process, an anode having a partially pre-polarized electrode which has been provided with a doped diamond layer is used.

BACKGROUND

1. Technical Field

The present invention relates to the electrochemical production ofperoxo-disulfuric acid with the use of diamond-coated electrodes.

2. Related Art

With a normal potential (E_(o)) of 2.01 V, peroxo-disulfuric acid(H₂S₂O₈) is one of the strongest known oxidizing agents, which is usedin a wide variety of fields. The most important areas of application ofperoxo-disulfuric acid include etching processes in the electronicsindustry and the production of particular plastics, for example use inthe polymerization of acrylonitrile.

Peroxo-disulfuric acid also has applications in waste treatment, theoxidation of dyestuffs and the bleaching of fibers. In addition to this,peroxo-disulfuric acid is an important intermediate product for theelectrochemical production of hydrogen peroxide.

The mechanism of the formation of peroxo-disulfuric acid by the anodicoxidation of sulfuric acid is complex. It is assumed that it comprisesthe formation of hydroxyl radicals. According to the mechanism water isfirst of all discharged at the anode with the formation of adsorbedhydroxyl radicals (see Equation 1). The hydroxyl radicals, which arepresent adsorbed at the diamond surface, react with the hydrogen sulfateions (see Equation 2) contained in the electrolyte, which form theactual peroxo-disulfuric acid in a subsequent dimerization step (seeEquation 3).

 H₂O→OH.+H⁺+ε⁻  (1)HΣO₄ ⁻→HSO₄ .+e ⁻  (2)(2 HSO₄.→H₂S₂O₈).  (3).

High concentrations of sulfuric acid and high current densities arerequired in this case, because when dilute solutions and small currentdensities are present, the low concentration of discharged sulfate ionsresults in the latter not reacting with one another (see Equation 3),but with the water, with the formation of oxygen:SO₄+H₂O→H₂SO₄+½O₂  (4)

There may furthermore be formed as by-products: oxygen by thedecomposition of water, ozone, peroxo-monosulfuric acid and hydrogenperoxide, according to the following (see Equations 5 and 6:H₂S₂O₈+H₂O→H₂SO₅+H₂SO₄  (5)H₂SO₅₊H₂O→H₂SO₄+H₂O₂  (6).

The effectiveness of the electrochemical peroxo-disulfuric acidproduction depends substantially on the electrode material used, ofwhich high requirements are made because of the prevailing oxidative andcorrosive conditions.

For example, the electrode material must be corrosion-resistant andstable against anodic dissolution.

Furthermore, the peroxo-disulfuric acid formation takes place in apotential range in which water is already decomposed with the productionof oxygen. In order to suppress the competing oxygen production,therefore, the electrode material must exhibit a high overvoltage forthe reaction.

Flat, large-area platinum electrodes are currently used for thelarge-scale electrochemical production of peroxo-disulfuric acid, withhigh sulfuric acid concentrations and high current densities. However,the platinum electrodes are gradually dissolved in the course of thereaction, so that the corrosion products obtained have to be removed bycomplicated cyclical means.

In order to obtain a satisfactory yield, a highly-concentrated sulfuricacid solution containing 7.5 moles must furthermore be used aselectrolyte. Highly-concentrated sulfuric acid solutions of this kindmay now, however, because of the oxidative and corrosive properties, behandled in special units and are therefore expensive in equipment terms.

Because of the expensive equipment required, peroxo-disulfuric acid isproduced in plants specially equipped for it and has to be procured fromthere. It would be desirable, however, for peroxo-disulfuric acid to beable to be produced as required directly on site, that is to say at theplace of use, since peroxo-disulfuric acid is because of its extremelyreactive properties difficult to store and in addition freeperoxo-disulfuric acid is subject to rapid hydrolysis in aqueoussolution.

Just recently, diamond-coated electrodes have because of their highchemical stability been attracting increasing interest for applicationsin electrochemical processes.

Such electrodes, in which a boron- or nitrogen-doped diamond layer isapplied to a suitable support material, may be obtained in the main bymeans of the known CVD (Chemical Vapor Deposition) technique.

For example, EP 0 714 997 B1 discloses the use of an electrode of ametal-containing substrate, in particular titanium, to which aboron-doped diamond layer has been applied, for the oxidation of spentphotographic baths and in the electronics or optoelectronics sectors.

It has been found, however, that the adhesive strength of diamond layerson metal-containing support materials such as titanium is notsatisfactory.

To improve the adhesiveness, therefore, in EP 0 730 043 A1 anintermediate layer is provided between the support material and thediamond layer, which consists of the decomposition products of ametallocene, preferably biscyclo-pentadienyltitanium chloride.

The production of diamond-coated electrodes with silicon as supportmaterial for small areas of not more than 1 cm² is described for exampleby G. M. Swain in: Adv. Mater. 6 (1994), p.388.

In a number of papers, moreover, a very great potential range isreported for diamond electrodes, in which no water decomposition andhence oxygen production occurs (H. B. Martin, A. Argoitia, U. Landau, A.B. Anderson, J. C. Angus: J. Electrochem. Soc. 143 (1996) L 133; F.Beck, H. Krohn, W. Kaiser, M. Fryda, C. P. Klages, L. Schäfer:Electrochimica Acta 44 (1998) 525).

However, there occurs also there, in the potential ranges favorable forthe electrochemical production of peroxo-disulfuric acid, a significantevolution of oxygen, so that a suitability in principle of theelectrodes for a peroxo-sulfuric acid production in large, economicallysignificant yields-specifically also with low sulfuric acidconcentrations-was not able to be assumed.

It is also reported that ozone may be obtained with diamond electrodeshaving silicon as support, (A. Perret, W. Haenni, P. Niedermann, N.Skinner, Ch. Comninellis, D. Gandani: Electrochemical SocietyProceedings, Volume 97-32 (1997) 275).

The diamond-coated electrodes described above exhibit in general thedisadvantage that either the diamond layer may be deposited only onsmall surfaces (G. M. Swain op. cit.) or, as disclosed in EP 0 730 043A1, electrochemically stable electrodes with sufficiently firmlyadhering diamond layers may be obtained only with the use of a speciallyapplied intermediate layer.

The object of the present invention is to provide a process for theelectrochemical production of peroxo-disulfuric acid andperoxo-disulfates, with which peroxo-disulfuric acid or theperoxo-disulfate may be obtained in economically significant yields on alarge scale, including with low sulfuric acid concentrations.

The object according to the invention is achieved by a process in whichperoxo-disulfuric acid and peroxo-disulfates are producedelectrochemically with the use of electrodes coated with doped diamond.

Surprisingly, it was found according to the invention that contrary toexpectations electrodes coated with doped diamond are extremely wellsuited to the electrochemical production of peroxo-disulfuric acid orperoxo-disulfates.

Below the term “peroxo-disulfuric acid” will be used collectively forthe compounds peroxo-disulfuric acid and peroxo-disulfates produced.

In particular, if such electrodes are used, sufficiently high yields ofperoxo-disulfuric acid may still be obtained even with low sulfuric acidconcentrations. The finding is completely contrary to the prevailingview, according to which a high sulfate ion concentration is essentialfor obtaining high yields and avoiding subsidiary reactions.

Below the electrodes coated with doped diamond will also be referred toby the shortened form “doped diamond electrodes”.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will be explained in detail below with reference to thefigures, where:

FIG. 1 shows diagrammatically the layout of a preferred embodimentaccording to the invention of a doped diamond electrode;

FIG. 2 a diagram in which the dependence of the effectiveness of thedoped diamond electrodes used according to the invention on the sulfuricacid concentration and the current density is represented;

FIG. 3 a cyclogram of a preferably used electrode according to theinvention, which has not been subjected to a complete oxidativepre-treatment;

FIG. 4 diagrammatically the electrolytic cell used according to theexample; and

FIG. 5 a further diagram with the H₂S₂O₈ concentration as a function ofthe current density.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

If doped diamond electrodes are used, satisfactory yields are obtainedeven with sulfuric acid concentrations as low as only 0.1 mole.

For the process according to the invention, the concentration of thesulfuric acid solution is preferably set in a range of 0.1 mole to 7.5mole, in particular 1 mole. If the concentration is less than 0.1 mole,the yields become uneconomic. Although the doped diamond electrodes usedaccording to the invention are because of their high stability andelectrochemical properties suitable in principle for use in highlyconcentrated sulfuric acid solutions, a sulfuric acid solutioncontaining more than 7.5 mole may nevertheless be handled industriallyonly with difficulty.

A current density suitable for the process according to the inventionlies in a range of 10 mA/cm² to 5000 mA/cm², in particular 100 mA/cm² to1000 mA/cm², preferably 100 mA/cm² to 400 mA/cm².

In general it may be stated that the effectiveness rises with increasingconcentration of the sulfuric acid solution and increasing currentdensity. The dependence of the effectiveness on the sulfuric acidsolution concentration and the current density is shown in FIG. 2. InFIG. 2 the loading in Ah/dm³ is plotted on the right and the amount ofthe theoretically possible conversion rate in % at the top. Six measuredvalues are entered for each of three series of measurements. Circlesrepresent measurements with 0.1 m H₂SO₄ at 30 mA/cm², squares representmeasurements with 7.5 m H₂SO₄ at 30 mA/cm² and triangles representmeasurements with 7.5 m H₂SO₄ at 200 mA/cm².

Thus there is achieved with the process according to the invention withthe use of doped diamond electrodes having an H₂SO₄ concentration of 1mole and a current density of only 30 mA/cm² an effectiveness of 47%,which may be increased to up to 75% if the concentration is increased to7.5 mole and the current density to 200 mA/cm².

The diamond electrodes used for the process according to the inventionmay be of any form. Plate, expanded metal, lattice or mesh electrodesmay be used. A so-called expanded metal form is particularly suitablefor large-scale plants. Advantageous properties are thereby utilized,such as good electrolyte exchange, economical use of expensive basemetals and a largely homogeneous current output due to homogeneouslydistributed preferred areas for the anode reaction such as tips andedges. In addition, the form may be coated particularly reliably. Theelectrode form is particularly suitable also for electrolyte solutionswith low H₂SO₄ concentration.

Sintered plate electrodes, which may be porous or dense, may be used asplate electrodes.

According to a particular embodiment it is also possible to useso-called three-dimensional electrodes such as ball electrodes. Ballelectrodes may be formed of a multiplicity of coated ball-shapedelectrodes, which are swept by the electrolyte in the manner of afluidized bed.

The cell type is also not subject to any particular limitations.Monopolar or bipolar cells with or without separation or subdivision ofthe electrode chambers by, for example, ion-selective membranes may beused.

A separation of the electrode chambers by, for example, ion-selectivemembranes is however to be recommended in order to prevent a cathodicreaction of the peroxo-disulfuric acid formed. The yields may beimproved still further by such a measure.

Particularly suitable for the process according to the invention aredoped diamond electrodes such as are described in the subsequent printedpublications DE 198 42 396 A1 and EP 0 994 074 A2 of the applicant, towhich reference is explicitly made here.

Using the measures described there for the known gas-phase depositionprocess (CVD), sufficiently large support materials and ones withcomplex shapes (hereinafter called “base bodies”) may be coated withcontinuous, easily adhering diamond layers.

Thus the homogeneous coating of sufficiently large areas up to a fewthousand cm² in size is possible with the process.

It has been found that with the process described there electricallyconductive diamond layers with a specific resistance of an order ofmagnitude of between 0.005 and 10 Ωcm may be deposited directly on ametallic, graphite or ceramic base material in an electrochemicallystable manner.

Examples of suitable metallic base materials are niobium, tantalum,titanium and zirconium, wherein tantalum is particularly preferred.Examples of suitable ceramic base materials are silicon, siliconcarbides such as silicon-filtered SiSiC or SiC, and silicon nitride,which exhibit a sufficient conductivity.

Preferably use is made for the base material of a self-passivatingmaterial, in particular a self-passivating metal, whereby impairment ordamage of the electrode or the base material due to electrolyte solutionwhich could penetrate into the electrode interior through pores orcracks possibly appearing in the deposited layer during the gas-phasedeposition is prevented. Examples of such self-passivating metals arethe aforementioned elements titanium, niobium, tantalum or zirconium, aswell as alloys of the materials or else other self-passivating metals.Titanium, however, is the first choice on cost grounds.

According to the invention the diamond layer may preferably be dopedwith boron, nitrogen, phosphorus or sulfur, wherein boron and nitrogenare particularly preferred. The content of boron may lie between 0.05ppm or 10 ppm and 10,000 ppm, preferably between 0.05 ppm and 100 ppm.The content of nitrogen may lie between 5 ppm and 100 ppm.

The diamond electrodes described in the above-mentioned subsequentprinted publications DE 198 42 396 A1 and EP 0 994 074 A2 arecharacterized by a particularly high adhesion of the diamond layer tothe base material. It is assumed that the exceptional adhesion is causedby the process-controlled formation of a metal carbide layer at theso-called interface, the transition area between the base material andthe layer of diamond, whereby a significantly improved mechanicalstability is obtained.

According to the invention it was furthermore established that animprovement in adhesion may also be improved by a carbonitride layer atthe interface, wherein particularly good results are observed in thiscase with ceramic base materials.

In a particularly preferred embodiment, such as that shown in FIG. 1,the electrode may be formed as a composite material electrode, whereinthe center 1 of the electrode is formed e.g. of a copper or aluminumcore which is characterized by a particularly high conductivity andrelatively low costs. The center 1 is covered with a dense envelope 2 ofa preferably self-passivating metal, in particular titanium. Theelectrically conductive doped diamond layer 3 may then be deposited onthe envelope 2.

The center 1 and the envelope 2 together form the base material 1, 2, onwhich the electrically conductive diamond layer 3 is deposited.

Between the diamond layer 2 and the surface area of the envelope 2, theinterface area, is located a carburized metal layer 4 which in theaforementioned example consists of titanium carbide.

Below will be described in principle a gas-phase deposition process,taking as an example the so-called Hot Filament CVD technique, forproducing the preferably used electrodes.

Use will generally be made, for the gas-phase deposition of a diamondlayer on the base material, of a gas mixture which contains a carbonsource, hydrogen and a source for the doping means, which according tothe example described here is a boron source.

A preferred carbon source is methane and a preferred boron sourcetri-methylborate, wherein the compounds are preferably used in the ratioof 1:1. Trimethyl borane may also be used in an amount of 0.05 ppm to100 ppm.

The boron content of the diamond layer may be adjusted by means of theboron content in the gas phase.

For the present invention the figures for the quantitative proportion ofthe individual components in the gas phase refer to the volume.

In the preferred embodiment the gas phase consists of 95% to 99.9%hydrogen (H₂); 0.1% to 5% methane (CH₄); and trimethylborate with acontent of about 1 ppm to 1%, wherein the trimethylborate: methane ratiodoes not exceed 1:1. In the preferred embodiment, the hydrogenconcentration preferably ranges from 95% to 99% and the methaneconcentration ranges preferably from 0.5% to 1%.

A smaller or higher proportion of carbon source may be selecteddepending on the nature of the carbon source used. For methane aproportion of about of 0.5% to 2% in the gas phase has proved to beparticularly advantageous. If the proportion is smaller, the growth ratebecomes uneconomic, and if the proportion is too high, the quality ofthe layer obtained suffers.

It should be borne in mind that the trimethylborate or trimethylboraneused as boron source represents simultaneously a further carbon source.

The process pressure is set at 5 to 50 hPa, but may also come to up to300 hPa if required.

The temperature of the heating or glow wires used (also termed“filaments”) comes as a rule to 2000° C. to 2400° C., where it may inparticular for electrodes with ceramic base material also be up to 2800°C. A high activation of the gas phase for the coating process is therebyobtained. Care is however taken on the substrate side that, depending onthe material, temperatures of 600° C. to 950° C. are not exceeded.

The setting of the substrate temperature may also be carried out byadapting the filament diameters, the filaments spacings and/or thefilament-substrate distance. External heating or cooling may also beused.

The content of boron in the diamond layer comes preferably to between 10ppm and 10 000 ppm, it may therefore come to up to 1%, wherein the boroncontent in the diamond layer as a rule lies far below 1%.

Doped diamond layers with a thickness of between 0.5 μm and 50 μm may beobtained with the process described. If the base material is notceramic, somewhat thicker layers are preferred, for example with athickness of preferably 2 μm to 50 μm, wherein however smallerthicknesses are also possible.

The carburizing at the boundary layer (interface) between base material1, 2 and diamond layer 3 deposited thereon may for example be carriedout in advance prior to the actual deposition of the diamond layer oralternatively be incorporated in the gas-phase deposition process.

In the first alternative the surface carburizing of the base materialmetals takes place by their being heated to the process temperature indiscrete steps in the presence of hydrocarbon and hydrogen.

If the base material metals are coated without prior separatecarburizing according to the second alternative, the presence of methaneand optionally trimethylborate in the gas phase results in metal carbidealso being obtained in the interface area by virtue of chemicalreactions, until, as a consequence of the simultaneously occurringdeposition of diamond and the thereby occasioned insulation of theexisting metal surface from the methane and the trimethylborate, themetal carbide formation is completed.

A mixed form consisting of both alternatives is also possible for thecarburizing. However, the separate carburizing is particularlypreferred, since it permits a more targeted control of the process.

If a nitride layer is to be formed as intermediate layer, first andforemost a nitrogen source, preferably nitrogen as such, is added asreactive gas, which reacts with the base material surface, in this casepreferably a ceramic base material, with the formation of nitrides.

For the gas-phase deposition the base material, consisting of the center1 with the copper or aluminum core and the envelope 2 consisting of thepreferably passivating metal, is roughened at the surface, for exampleby sand or ball blasting. The roughening serves to support the adhesion.Thereafter a pre-seeding in a suspension of nanodiamond and 0.5 μmdiamond powder in ethanol takes place.

According to a particularly preferred embodiment, doped diamondelectrodes are used which have not been subjected to a completeoxidative pre-treatment prior to initial start-up.

For the purpose of the invention the term “complete oxidation” meansthat the surface of the electrode, which is hydrogenated in theuntreated state, is oxidized up to the highest possible oxidation state,it being assumed that carbonyl groups are thereby formed.

Diamond electrodes which have been subjected to such an anodicpre-treatment or polarization are generally considered to beparticularly stable and are said to behave unchanged electrochemicallyover a very long period. Theoretical discussions of this are found in H.B. Martin, A. Arguitia, U. Landau, A. B. Anderson, J. C. Angus, in: J.Electrochem. S.o.k. 143 (1996) L. 133.

Experimental tests have shown, however, that although the desiredstabilized state of the electrodes may be achieved by pre-polarization,this is nevertheless at the cost of the intensity and the effectiveness.

Surprisingly, it was found that even non-pre-polarized diamondelectrodes, or ones only partially pre-polarized, exhibit a sufficientstability for the production of peroxo-disulfuric acid.

If non-pre-polarized diamond electrodes or partially pre-polarized onesare used, the voltage must be held in a range in which completepre-polarization does not take place. The process is therefore alwaysoperated in the potential range below that voltage at which oxygen mayevolve, i.e. polarization occurs. In order nevertheless to obtain thehighest possible levels of effectiveness, the voltage should however bekept as close as possible below the potential range.

A closer investigation of the effect showed that the effectiveness ofthe electrode may be increased if prior to actual start-up it is chargeddeliberately with a load of approximately 0.1 C/cm² electrode surfacefor partial oxidation.

“Partial oxidation” means for the purpose of the invention that theoxidation is stopped at a lower oxidation state than that which is setfor complete pre-polarization. It is assumed that in so doing hydroxylgroups form at the electrode surface.

The effectiveness increases up to approximately the load, until withfurther loading it again decreases significantly and finally drops belowthe original value as previously described.

In FIG. 3 the unexpected behavior of diamond electrodes is representedby means of a cyclogram. On the right the potential is plotted in voltsversus a standard hydrogen electrode (SHE), at the top the currentdensity in A/cm². The temperature prevailing at the start of thecyclogram came to 25° C., the counter-electrode consisted of platinumand 1N H₂SO₄ was used as electrolyte. The measuring speed came to 200mV/s.

The cyclogram shows in the solid line the behavior of a diamondelectrode after it has been pre-polarized. An oxidative pre-treatmenthas occurred here, therefore, for example by the application of a veryhigh voltage over a prolonged period.

The other dashed lines of various kinds show the behavior of the diamondelectrode preferably used according to the invention without or with apartial pre-polarization. Already during the first cycle a small maximumappears in the range around about 2.2 V, or a turning point at about2.35 V.

The maximum points to an electrochemical reaction. Only at a stillhigher voltage does the expected maximum in current density appear dueto the increasing evolution of oxygen.

If the same electrode is now subjected to a further cycle, the maximumsoon becomes very clear at about 2.2 V and the subsequent minimum at 2.2V is clearly discernible. The trend continues with further cycles. Itbecomes strongest in the 6th cycle, in which the maximum has hereshifted to about 2.4 V and the subsequent minimum now also occurs at ahigher voltage value of about 2.65 V.

In still further cycles the value of the maximum then again decreasessignificantly. The curve for the 10th cycle is still represented. Themaximum shifts further to higher voltage values, but declines in itslevel. The subsequent minimum also shifts to higher voltage values. Thetrend continues in further cycles not shown here in the figure.

The non-reversible, electrochemical reaction therefore initiallyincreases in intensity with each cycle, and then decreases again. In thelast analysis the solid black line is at the same a trend-likedevelopment which is also aimed for as a maximum value in furthercycles.

It is clearly discemible in turn that the electrochemical reactioncontinues to occur only with low intensity after the pre-polarization.The formation of highly reactive oxygen compounds is involved in thereaction.

The reaction also takes place on other electrode materials, but onlywith the simultaneous formation of oxygen and hence with far lowerefficiency.

In practice the potential will not be applied to the electrodes in theform of one or more cycles, as this is very expensive.

It is nevertheless possible by means of the cycles to determine veryprecisely for a particular electrode type the applied load at which itoperates most effectively, that is to say the load from which theeffectiveness decreases again.

Considered in detail, the load per unit of area of the electrode surfaceis involved here. The optimum load amount per unit of area varies,however, with different electrode types. This is because the surfacestructure, i.e. the crystal orientation or else the form of theelectrode, for example, influences the maximum range.

If however the load content is determined experimentally for aparticular electrode type by subjection to a plurality of cycles, theload may be applied specifically for further electrodes of the sametype, i.e. not in a plurality of cycles, but by corresponding chargingover time to each individual electrode of precisely the load.

It has been found experimentally that the optimum range lies atapproximately 0.01 to 1, in particular at approximately 0.1, Coulomb persquare centimeter (C/cm²).

The background to this unexpected effect, which has not been reported todate in publications, probably lies in the fact that the loadapplication promotes the formation on the crystal surface of OH groups,which then increase the effectiveness of the electrode. If a greaterload than for maximum effectiveness is applied, the OH groups possiblystart to react with one another and thereby reduce the effectivenessagain after reaching the maximum.

It becomes clear from this investigation that the production of reactiveoxygen components such as peroxo-disulfuric acid on such diamondelectrodes may be influenced advantageously by dispensing with theconventional, that is to say complete pre-polarization. The presentprocess will be illustrated below by means of an example.

EXAMPLE 1

Production of an Electrode with a Boron-doped Diamond Layer

A boron-doped diamond layer was produced by means of HF-CVD (HotFilament Chemical Vapor Deposition) technique on monocrystalline p-Si(100) wafers (0.1 Ωcm, sold under the name Siltronix).

The temperature of the filaments lay in a range of 2440° C. to 2560° C.,and the substrate was held at 830° C. Methane was used as a reactive gasin an excess of hydrogen (1% methane in H₂). Trimethylborane in aconcentration of 3 ppm was used for the doping. The gas mixture wasadded to the reaction chamber at a flow rate of 5 dm³/min, wherein agrowth rate of 0.24 μm/h was obtained for the diamond layer. The diamondlayer obtained had a thickness of about 1 μm. Columnar, randomlytextured polycrystalline layers were obtained.

EXAMPLE 2

Production of Peroxo-disulfuric Acid

Peroxo-disulfuric acid was produced using electrodes obtained accordingto Example 1. The production took place in a single-cell electrolyticflow cell A (FIG. 4) with H₂SO₄ as electrolyte 7 with an electrolyteinlet 8 and an electrolyte outlet 9 together with electric connections10, 11. The diamond electrode was the anode 5 and zirconium the cathode6. Both electrodes were circular with a diameter of 80 mm and an area of50 cm² respectively. The distance between the electrodes came to 10 mm.A thermoregulated glass storage container with a capacity of 500 cm³ wasused for the electrolyte 7 and circulated through the cell A by means ofa pump.

The electrolysis was carried out under galvanostatic conditions and anelectrolyte temperature of 25° C. During the electrolysis theconcentration of the peroxo-disulfuric acid was determined by means ofiodometric titration and plotted as a function of the specific electriccharge (Ah/dm³) used (FIG. 5). The formation of peroxo-disulfuric acidwas confirmed by means of the specific Ni(OH)₂ test in the presence ofsilver nitrate in order to avoid disturbing reactions with otheroxidizing agents such as H₂O₂.

In order to prevent the electrochemical reduction of peroxo-disulfuricacid at the cathode and its hydrolysis to peroxo-monosulfuric acid, theelectrolysis took place with a low sulfuric acid conversion rate (<5%)and with short electrolysis times (<1 h).

The result is shown in FIG. 5 in which the load is plotted on the rightin Ah/dm³ and the concentration of H₂S₂O₈ at the top in mole/l. The fourseries of measurements entered then represent the following curves:

-   (a) sulfuric acid concentration: 1.0 mole/L, current density i=30    mA/cm-   (b) sulfuric acid concentration: 7.5 mole/L, current density i=30    mA/cm²-   (c) sulfuric acid concentration: 7.5 mole/L, current density i=200    mA/cm²-   (d) theoretical sulfuric acid concentration with an effectiveness of    100%; temperature respectively 25° C.    The theoretical value according to (d) was calculated for Faraday's    law and a normal potential E_(o)=2.01 V for SO₄ ²⁻/S₂O₈ ²⁻.

1. A process for the electrochemical production of peroxo-disulphuricacid and peroxo-disulphates by the electrochemical oxidation of sulfuricacid, comprising the step of: using an electrode coated with a dopeddiamond layer.
 2. The process of claim 1, wherein the sulfuric acidconcentration of the electrolyte ranges from 0.1 mole to 7.5 mole. 3.The process of claim 1, further comprising the step of: carrying out theelectrolysis using a current density ranging from 10 mA/cm² to 5000mA/cm².
 4. The process of claim 1, further comprising the step of: usinga monopolar cell as the electrolytic cell.
 5. The process of claim 4,further comprising the step of: subdividing the electrode chamber of thecell.
 6. The process of claim 5, wherein the electrode chamber of thecell is subdivided by an ion-selective membrane.
 7. The process of claim1, further comprising the step of: selecting the electrode form from thegroup consisting of a plate electrode, an expanded metal electrode, alattice electrode, a mesh electrode and a three-dimensional electrode.8. The process of claim 7, further comprising the step of: selecting asintered plate electrode.
 9. The process of claim 8, wherein the plateelectrode is a porous sintered plate or a dense sintered plate.
 10. Theprocess of claim 1, wherein the anode comprises a large-area layer ofdoped diamond consisting of a base material.
 11. The process of claim10, further comprising: an intermediate layer positioned between thebase material and the layer of diamond.
 12. The process of claim 11,wherein the intermediate layer is a carburized intermediate layer. 13.The process of claim 12, wherein the carburized intermediate layer is ametal carbide layer or a carbonitride layer.
 14. The process of claim11, wherein the intermediate layer is formed from a material that isselected from the group consisting of titanium, niobium, tantalum,zirconium, and alloys thereof, silicon, silicon carbide,silicon-filtered silicon carbide (SiSiC), and silicon-based ceramic. 15.The process of claim 10, further comprising the step of: doping thelayer of diamond with an element selected from the group consisting ofboron, nitrogen, phosphorus, and sulfur.
 16. The process of claim 15,further comprising the step of: doping the layer of diamond with boron.17. The process of claim 16, wherein the boron content in the layer ofdiamond ranges from 10 ppm to 10,000 ppm.
 18. The process of claim 15,wherein the nitrogen content in the layer of diamond ranges from 5 to100 ppm.
 19. The process of claim 1, wherein the doped diamond layer ispartially polarized.
 20. The process of claim 19, further comprising thestep of: charging the electrode surface with a load of 0.01 C/cm² to 1C/cm² before the step of electrolysis.
 21. The process of claim 19,further comprising: performing the electrolysis below the potentialrange at which a substantial oxygen evolution occurs at the electrode.22. The process of claim 21, wherein the voltage at the electrode liesduring operation precisely in the potential range at which an oxygenevolution starts.
 23. The process of claim 19, further comprising thestep of: charging the electrode surface with a load of 0.05 C/cm² to 0.2C/cm² before the step of electrolysis.
 24. The process of claim 1,further comprising the step of carrying out the electrolysis using acurrent density ranging from 100 mA/cm² to 1000 mA/cm².
 25. The processof claim 1, further comprising the step of: using a bipolar cell as theelectrolytic cell.
 26. The process of claim 25, further comprising thestep of: subdividing the electrode chamber of the cell.
 27. The processof claim 26, wherein the electrode chamber of the cell is subdivided byan ion-selective membrane.
 28. A method of using an electrode for theelectrochemical production of peroxo-disulfuric acid andperoxo-disulfates, the method comprising: using an electrode coated witha doped diamond layer.
 29. A cell for producing peroxo-disulfuric acidand peroxo-disulfates, the cell comprising: a housing; an electrodecoated with a doped diamond layer, the electrode having a substantiallyhydrogenated surface; a counter-electrode having a substantiallyhydrogenated surface; and a sulfuric acid electrolyte; wherein theelectrode and counter-electrode are arranged in the housing.
 30. Thecell of claim 29, wherein the electrode chamber is subdivided.